LiF Protective Layer on a Li Anode: Toward Improving the Performance of Li-O2 Batteries with a Redox Mediator

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Amphi-Active Superoxide-Solvating Charge Redox Mediator for Highly Stable Lithium-Oxygen Batteries

A multifunctional electrolyte additive for lithium oxygen batteries (LOBs) was designed to have (1) a redox-active moiety to mediate decomposition of lithium peroxide (Li₂O₂ as the final discharge product) during charging and (2) a solvent moiety to solvate and stabilize lithium superoxide (LiO₂ as the intermediate discharge product) in electrolyte during discharging. 4-Acetamido-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) or AAT was employed as the additive working for both charge and discharge processes (amphi-active). The redox-active moiety was rooted in TEMPO, while the acetamido (AA) functional group inherited the high donor number (DN) of N,N-dimethylacetamide (DMAc). Integrating two functional moieties (TEMPO and AA) into a single molecule resulted in the bifunctionality of AAT (1) facilitating Li₂O₂ decomposition by the TEMPO moiety and (2) encouraging the solvent mechanism of Li₂O₂ formation by the high-DN AA moiety. Significantly improved LOB performances were achieved by the superoxide-solvating charge redox mediator, which were not obtained by a simple cocktail of TEMPO and DMAc.

A Low-Volatile and Durable Deep Eutectic Electrolyte for High-Performance Lithium-Oxygen Battery

The lithium-oxygen battery (LOB) with a high theoretical energy density (∼3500 Wh kg^-1) has been regarded as a strong competitor for next-generation energy storage systems. However, its performance is still far from satisfactory due to the lack of stable electrolyte that can simultaneously withstand the strong oxidizing environment during battery operation, evaporation by the semiopen feature, and high reactivity of lithium metal anode. Here, we have developed a deep eutectic electrolyte (DEE) that can fulfill all the requirements to enable the long-term operation of LOBs by just simply mixing solid N-methylacetamide (NMA) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a certain ratio. The unique interaction of the polar groups in the NMA with the cations and anions in the LiTFSI enables DEE formation, and this NMA-based DEE possesses high ionic conductivity, good thermal, chemical, and electrochemical stability, and good compatibility with the lithium metal anode. As a result, the LOBs with the NMA-based DEE present a high discharge capacity (8647 mAh g^-1), excellent rate performance, and superb cycling lifetime (280 cycles). The introduction of DEE into LOBs will inject new vitality into the design of electrolytes and promote the development of high-performance LOBs.

Ammonium Ionic Liquid-Functionalized Phenothiazine as a New Redox Mediator for High Chemical Stability on the Anode Surface in Lithium-Air Batteries

The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium-air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium-based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion-exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs.

Strategies toward anode stabilization in nonaqueous alkali metal-oxygen batteries

Alkali metal-O2 batteries exhibit ultra-high theoretical energy density which is even on a par with to fossil energy and expected to become the next generation of energy storage devices. However,...

Stabilizing Li Anodes in I2 Steam to Tackle the Shuttling-Induced Depletion of an Iodide/Triiodide Redox Mediator in Li-O2 Batteries with Suppressed Li Dendrite Growth

Redox mediators (RMs) have become a significant point in the now-established Li-O₂ battery system to reduce the charging overpotential in the oxygen evolution process. Nevertheless, a major inherent barrier of the RM is the redox shuttling between the Li metal anode and mobile RM, resulting in the corrosion of Li and depletion of RM. In this study, taking iodide/triiodide as a model RM, we propose an effective strategy by immersing the Li metal anode in I₂ steam to create a 1.5 μm thick surface protective layer. The resultant ionic conductive LiI layer on the Li metal anode can not only suppress Li dendrite growth but also act as a buffer layer between the RM and bare Li. By combining the iodide/triiodide RM with the LiI protective layer, the Li-O₂ battery shows low and steady charge voltage plateaus of ∼3.6 V over 70 cycles. Importantly, the symmetrical cell using the LiI-protected Li electrode exhibited small Li plating/stripping overpotentials (∼20 mV, 480 h), far superior to that of the bare Li electrode (∼70 mV, 300 h). The in situ interfacial observation shows that dendrite growth on the Li metal can be effectively suppressed by optimizing the LiI protective layer.

Wax-Transferred Hydrophobic CVD Graphene Enables Water-Resistant and Dendrite-Free Lithium Anode toward Long Cycle Li-Air Battery

One of the key challenges in achieving practical lithium-air battery is the poor moisture tolerance of the lithium metal anode. Herein, guided by theoretical modeling, an effective tactic for realizing water-resistant Li anode by implementing a wax-assisted transfer protocol is reported to passivate the Li surface with an inert high-quality chemical vapor deposition (CVD) graphene layer. This electrically conductive and mechanically robust graphene coating enables serving as an artificial solid/electrolyte interphase (SEI), guiding homogeneous Li plating/stripping, suppressing dendrite and "dead" Li formation, as well as passivating the Li surface from moisture erosion and side reactions. Consequently, lithium-air batteries fabricated with the passivated Li anodes demonstrate a superb cycling performance up to 2300 h (230 cycles at 1000 mAh g-1 , 200 mA g-1 ). More strikingly, the anode recycled thereafter can be recoupled with a fresh cathode to continuously run for 400 extended hours. Comprehensive time-lapse and ex situ microscopic and spectroscopic investigations are further carried out for elucidating the fundamentals behind the extraordinary air and electrochemical stability.

Organic-Inorganic Hybrid SEI Induced by a New Lithium Salt for High-Performance Metallic Lithium Anodes

The practical application of the metallic lithium anode is suppressed by the highly unstable interface between electrolytes and lithium metal during the process of lithium plating/stripping. A perfect solid electrolyte interphase (SEI) can inhibit detrimental parasitic reactions, thereby improving the cycling performance of the metallic lithium anode. In this work, a high-purity solid lithium difluorobis(oxalato) phosphate (LiDFOP) is synthesized and an outstanding organic-inorganic hybrid SEI is obtained in an ether-based electrolyte for the first time induced by LiDFOP. The preferential reduction of LiDFOP can form an SEI rich in LiF and Li_xPO_yF_z species, thereby improving the conductivity and stability of the SEI. In addition, cationic-induced ring-opening polymerization between LiDFOP and 1,3-dioxolane endows the SEI with excellent adaptability to the reiterative volume change of the metallic lithium anode. Therefore, the Li/Cu battery maintains a high coulombic efficiency of 98.37% at a current density of 2 mA/cm² for 200 cycles, and the Li/Li symmetrical battery shows stable voltage hysteresis over 1000 h even under the condition of 5 mA/cm². The Li/S battery fabricated employing the electrolyte with LiDFOP shows significant improvement of cycling performance as well. These results manifest that the formation of an organic-inorganic hybrid SEI from LiDFOP can be employed as a new strategy to overcome the problem from the unstable SEI in metallic lithium batteries.